1. Field of the Invention
The present invention relates to an apparatus for detecting particles that may be stuck on the surfaces of substrates, for example, reticles and masks used for producing printed circuit patterns on semiconductor wafers or substrates that are used in liquid crystal displays and, more particularly, to a self-correcting or calibrating feature which can compensate for any errors in the detecting apparatus.
2. Description of Related Art
An apparatus for detecting particles in which an inspection stage receives a substrate to be inspected is known. An inspection stage is adapted to linearly move between a particle-detecting position or station and a subsequent particle observing position or station. In the particle-detecting position, a laser beam is applied at a predetermined angle to the surface of the substrate. The laser beam can have a polarized characteristic and can be scanned across the substrate by a revolving beam-scanning mirror. Any reflected and/or scattered incident laser beams that have been reflected or scattered from the particles and the like on the surface of the substrate will be detected by a detector optical system that is positioned adjacent to the substrate and is capable of recording variations in the intensity of reflected or scattered laser beams. The existence of a minute particle on the substrate surface can have the characteristic of reflecting and scattering the laser beam, and will vary the intensity of the detected signal from the detector optical system to indicate a measurement of the size of the particle, and also the position of the particle on the substrate. The substrate can then be moved to the particle observing position, and a correlation between the detected positions of the particles can be used to align the substrate for visual observation and confirmation by the operator with the use of a microscope. Thus, minute particles can be efficiently observed if accurate particle positioning information can be provided.
In the operation of the conventional apparatus for detecting particles, there is a relationship between the angle of the swing signal of the beam-scanning mirror that reciprocally scans a laser beam, emitted from a laser cavity, and an angle of swing or position of a laser beam spot on the substrate, which is calculated on the basis of the angle of the swing signal from the beam-scanning mirror. Thus, the position of the laser beam is determined during the scan cycle.
However, there is a possibility that the laser beam as incident upon the substrate to be inspected may be in disagreement with the angle of the swing signal output from the beam-scanning mirror. Thus, the beam spot, due to characteristics of the actual optical detecting system such as the optical adjustment of the incident optical system and the accuracy of the optical elements, along with alignment features of the components, can produce an error between an apparent detected position and an actual position which requires realignment and readjustments of the microscope to hunt for the detected particle.
Referring to FIG. 5, this error is graphically disclosed. If there are no errors in the optical detecting system and it performed hypothetically in accordance with its design purpose, then a linear relationship would be maintained between the detected position and actual position of any minute particles, such as shown by the straight line I in FIG. 5. The line II discloses, in fact, the detected positions that the apparatus provides as a result of the various inaccuracies that are inherent in a production line optical detecting system.